This invention relates to a tuning fork-shaped quartz oscillator which oscillates in both a bending mode and a twisting mode at the same time.
There is known a wristwatch provided with a tuning fork-shaped quartz oscillator which generates signals having a resonance frequency of 32.768 KHz. The output signals of the quartz oscillator are frequency-divided to provide 1 Hz signals, which drive the movement of the watch. The quartz oscillator oscillates in a bending mode to produce signals of a relatively low resonance frequency, i.e. 32.768 KHz. Since its frequency is low, the output signal of the quartz oscillator can be frequency-divided by a frequency divider having a small number of frequency-dividing stages made of, for example a C-MOS integrated circuit which consumes little power.
The accuracy of a wristwatch provided with a quartz oscillator is determined by the frequency-temperature characteristic of the quartz oscillator. This characteristic is represented by a curve of secondary degree, the peak of which coincides with a temperature of about 20.degree. C. Any wristwatch provided with a quartz oscillator is so designed as to keep good time over a temperature range of -10.degree. C. to +60.degree. C. To enhance the accuracy of the watches it is required that the resonance frequency of the quartz oscillator used should remain unchanged over said temperature range. If provided with a tuning fork-shaped quartz oscillator of known type whose resonance frequency varies along with temperature, a wristwatch will gain or lose about 15 seconds every month and will not be said to be highly accurate.
AT-cut quartz oscillators which oscillate in a sliding mode are used also in wristwatches. They have a frequency-temperature characteristic which is represented by a curve of third degree having a horizontal portion. This means that the resonance frequency of an AT-cut quartz oscillator remains unchanged over a specific temperature range unlike the tuning fork-shaped quartz oscillator which oscillates in a bending mode. If provided with an AT-cut quartz oscillator whose resonance frequency is, for example, 4.19 MHz, a wristwatch will gain or lose only about five seconds per year.
The high resonance frequency, e.g. 4.19 MHz, of the output signals of an AT-cut quartz oscillator cannot be effectively divided by a C-MOS integrated circuit. This is because the C-MOS integrated circuit operates at a low speed. If a C-MOS integrated circuit is to be used, the AT-cut quartz oscillator has to be made thicker thereby to reduce its output resonance frequency. But it is impossible to reduce the resonance frequency to that of a tuning fork-shaped quartz oscillator, however thin the AT-cut quartz oscillator is made. Inevitably, to divide the resonance frequency of an AT-cut quartz oscillator, use must be made of a C-MOS integrated circuit having more frequency-dividing stages than a C-MOS integrated circuit which can effectively divide the resonance frequency of a tuning fork-shaped quartz oscillator. In other words, a C-MOS integrated circuit of large power consumption must be employed to divide the resonance frequency of an AT-cut quartz oscillator. Further, if an AT-cut quartz oscillator is made thick to reduce its output resonance frequency, the wristwatch using the oscillator will inevitably be thick.
An object of this invention is to provide a tuning fork-shaped quartz oscillator which is small-sized and highly reliable and which can be used in combination with a C-MOS integrated circuit of little power consumption to provide, for example, a wristwatch.